The invention concerns a measuring arrangement for determining the lift of a valve member including a stationary capacitive sensor structure, a movable capacitive sensor structure which is connected to the valve member and arranged coaxially with the stationary capacitive sensor structure and, during movement of the valve, moves in overlapping relationship with the stationary capacitive sensor structure.
With a variable control of gas exchange valves, the efficiency and the generation of noxious emissions of internal combustion engines can be substantially improved. For controlling the operation of the valves, electromagnetic actuators or other electrically controllable operating mechanisms which permit a variable valve control over a large range are used. In order to adapt the valve opening movement to the requirements of the cylinder charge change and in order to minimize wear and energy consumption, an additional control of the movement of the valve member is necessary. As the basic value for controlling such a movement the system uses a position signal which represents the position of the valve member at any point in time.
In addition to the gas change, the fuel supply control is important for the efficiency and the emission behavior of internal combustion engines. In this connection, not only the fuel amount, but also the injection timing is important. Consequently, it is also important, to appropriately control the movement characteristics of a valve member of the fuel injection apparatus, for example, the movement of a respective injection nozzle needle. Also, a suitable position signal is utilized for this purpose.
DE 44 38 059 A1 discloses an arrangement for measuring the lift movement of valves such as a gas change valve or a fuel injection valve for an internal combustion engine. The valves are provided with magnetic circuits of which each includes a magnetic field generating coil which is stationary with respect to the valve and a magnetic core, which is moveable together with the valve and whose movement offsets the magnetic circuit. If the magnetic coils are energized by an AC current, the movement of the magnetic core causes in the magnetic circuit an impedance change which is used as a criterion for the lift movement of the respective valve. If the magnetic coils an energized by DC current, the voltage induced in the coils is used as the criterion for the speed of movement of the respective valve. In order to maintain expenditures low and to compensate for temperature influences the magnetic field generating coils of two valves which are operated at different points in time are connected in oppositely acting branches of a bridge circuit. However, because of the hysteresis effects, inductive measuring procedures are usable only in a limited way.
DE 43 26 379 A1 discloses a measuring arrangement for measuring the lift of spring-loaded valves of internal combustion engines with a capacitive lift travel sensor which includes a stationary sensor part associated with the housing and another sensor part movable with the valve. The two sensor parts are arranged within the space surrounded by the valve coil spring concentrically around the valve shaft. The sensor part, which is stationary with the housing and the sensor part which is movable with the valve form a valve lift-dependent measuring capacitor, wherein the overlap of the sensor parts provides, by a change of the capacitance, a measurement signal which depends on the valve lift. There is no compensation for temperature influences.
DE 33 31 170 C1 discloses an apparatus for determining the travel distance of a piston of hydraulic pneumatic or hydropneumatic apparatus such as vibration dampers, gas springs and hydropneumatic springs operating in accordance with the tubular capacitor principle. Herein, a first axially movable electrode is formed by a hollow piston rod and a second stationary insulated electrode is supported so as to extend into the hollow piston rod and the capacitance between the two electrodes is measured. In the cylindrical interior of the electrode fixed in the cylinder bottom another stationary tube is provided which is spaced radially from the fixed electrode tube and forms an additional electrode forming, with the first stationary electrode, a capacitor with constant capacity. In this way, a capacitive semi-bridge is formed which comprises the variable capacitance (by movement of the piston rod) and the constant capacitance structure mounted on the cylinder bottom. This arrangement provides for an improved evaluation of the variable capacitance and, at the same time, for a pressure and temperature compensation by taking the ratio of the capacitances into consideration in a suitable electronic control unit. However, with the small radial dimensioning of lift valves of internal combustion engines such an arrangement is not really feasible in connection with internal combustion engines.
EP 0 336 022 A1 discloses a capacitive measuring apparatus in which an electrode connected to a sensor cooperates with several other electrodes which are arranged co-axially and at the same distance from a center electrode. Of the capacitances formed in this way at least one has, during normal use, a constant capacity, whereas other capacitors change their capacity with a displacement of the sensor and generate a signal corresponding to the displacement of the sensor. In the preferred embodiment, a short center electrode is arranged between two end electrodes of the same size. The end electrodes form, with a coaxially arranged additional electrode, capacitors with variable capacitances.
It is the object of the present invention to provide an arrangement for generating a suitable position signal for valve members of an internal combustion engine which arrangement is simple and requires only a small space.